The recent development of hybrid powertrains in the automotive industry has demonstrated encouraging results for reductions in fuel consumption and exhaust emissions. A vehicle with a hybrid powertrain usually includes an internal combustion engine, an electric generator, an electric motor, a battery and other equipment. In series hybrid vehicles, the generator is driven by the mechanical output of the internal combustion engine. The output of the generator is then combined with the output of the battery to drive the electric motor, such that the mechanical output of the motor drives the vehicle. In contrast, the parallel hybrid vehicle includes an internal combustion engine, a regenerative brake/motor and an electric energy storage device such as a battery and other equipment. PHVs are usually driven directly by the mechanical output of the internal combustion engine. However, when the vehicle must be accelerated or decelerated at a rate that cannot be accomplished by the internal combustion engine alone, or if the drive efficiency of the engine would be degraded if only the internal combustion engine were used, the regenerative brake/motor, which is mechanically connected to the internal combustion engine, operates as an electric motor (on acceleration) or as a regenerative brake (on deceleration) to meet the required rate of acceleration or deceleration through the combined output of the internal combustion engine and the regenerative brake/motor.
The internal combustion engine of a hybrid powertrain has narrow operating range. In series hybrid vehicles, the internal combustion engine is not directly connected to the driving wheels while in parallel hybrid vehicles, the regenerative brake/motor provides rapid acceleration or deceleration. Therefore, the internal combustion engine used in hybrid powertrains can be optimized for better fuel economy and less exhaust emissions relative to powertrains that are solely powered by conventional internal combustion engines.
Examples of hybrid vehicles and their operating modes have been described in detail in several patents. For example, in U.S. Pat. No. 5,656,921, a parallel hybrid vehicle is disclosed having power sources from a SI (spark ignition) engine and an electric motor. It employs fuzzy logic rules to adjust the entries in the tables determining the power splitting between the SI engine and the electric motor. The performance measure used to adjust the entries is given by the weighted ratio between the battery current and fuel flow rate. In U.S. Pat. No. 5,943,918, granted to Reed and U.S. Pat. No. 6,164,400 granted to Jankovic, a hybrid powertrain is described which uses power delivered by both the internal combustion engine and the electric motor. A shifting schedule was developed for a multiple ratio transmission to establishing a proportional relationship between accelerator pedal movement and the torque desired at the wheel. U.S. Pat. No. 6,223,106 granted to Toru Yano et al. and U.S. Pat. No. 6,318,487 granted to Yanase et al. each describe a hybrid vehicle control system operable to prevent the battery from being overcharged during regenerating braking. U.S. Pat. No. 5,725,064, describes a control system operable to open the intake and exhaust valves to reduce the pumping loss when the vehicle is operating in reverse or its electric motor driving mode without using a clutch device to disconnect the internal combustion engine from the transmission. Finally, U.S. Pat. No. 6,266,956 describes an exhaust emission control system for a hybrid car using a separate combustion device to heat the catalyst and to provide hydrocarbons as the reducing agent to the lean NOx catalyst.
The primary focus of the above patents is the drivability of the hybrid vehicle. Unfortunately, little efforts have been applied to the development and integration of the internal combustion engines to optimize the benefits of the hybrid powertrain for lower cost, better fuel economy and lower exhaust emissions, especially, for the heavy-duty diesel engines for the urban and on-highway truck and bus applications.